A new high resolution method for visualizing and measuring the red cell velocity profile in small blood vessels has been discovered. To see the profile two pictures of blood flowing in a vessel are taken through a microscope a known, short time apart and viewed as a stereo pair. The displacement of the cells between the exposures is seen as an elevation or depression proportional to the displacement and measurable with a parallax bar. Collectively they form the velocity profile. The elevations and depressions are distinct, equal, and interchangeable, depending on how the information is presented to the eye and interpreted by the brain. One sees the profile in 3-D not just at one locus but all along the length of the vessel and its branches. If one knows the magnification, time between exposures, and the parallax measurement, one can calculate the velocity of individual cells, aggregates, and the profile. The two images may also be digitized and the displacement measured by computer-matching of the shifted density pattern. The method thus makes possible for the first time direct high resolution visualization and measurement of the velocity profile. It also provides a concise, high resolution, permanent record of all the data for study, publication, presentations, and storage. Our observations were made on a few selected scenes from a 1966 high-speed motion picture film kindly lent us by E.H. Bloch of Western Reserve School of Medicine. We propose to construct and optimize a modern, high-speed cinephotomicrographic system to take original data. Flow velocity or shear rate is a major physical variable in circulating blood but a neglected one because it is difficult to measure in sufficient detail in vivo. Cells are much more susceptible to damage from a tangential (shear) force than a perpendicular (blood pressure) one. Their effects (including that of viscosity), e.g., on the endothelium, may be interrelated and additive. We now have a good method for visualizing and studying this variable, and potentially its effects on blood and the vessel wall. We propose to study and extend the limitations normally imposed by the biological system on the method of stereo pairs, apply the method in selected studies of cell-cell, cell-plasma, and blood-endothelium interactions, and analyze the data by visual study of stereo pairs and measurement with a parallax bar and computer.